Vacuum tube circuits



Aug. 18,' 1931. D. F. WHITING 1,819,529

VCUUM TUBE CIRCUITS med Jan. ze, 192e Patented Aug. 18, 1931 srarssA DONALD F. WHITING, OF PORT WASHINGTON, NEW YORK, .A SSIGNOR TO BELL- TELE- PHONE II'JABORATORES IIINTIOREORATED,y OF NEW YORK, N. Y., A CORPORATION OF NEW YORK VACUUM TUBE CIRCUITS Application ledJanuary 29, 1926. Serial No.184,550.

This invention'relates to vacuum tube circuits and more particularly to inulti-stage amplilier circuits in which vacuum tubes arev employed in the various stages. Y

One of the objects of this invention is to provide means for regulating the degreeV of amplification of a multi-stage ainpliiier set for signals impressed thereon.

It is well-known in the art that weak currents or signals may be amplified to any desired degree by impressing them on a plurality ofvacuuintube ainpliiers arranged to work into each other by suitable circuit connections. In employing such a multistage amplifier set for amplifying weak currents, it is frequently found undesirable to use the total ainplilication of the set before impressing the ampliiied currents on the outgoing line or the receiving instrument, since the degree-of amplification desired for each particular case will depend largely on the strength of the incoming currents and o n the electrical conditions in the output circuit into which the amplifier works. It is also frequently desirable to cut down the degree of amplification to prevent overloading the last stages of the set and to prevent excessive distortion resulting therefrom.

It is customary in multi-stage tube circuits to utilize common circuits to supply the vvarious operating voltages to the various tubes. In such circuits, if a tube burnsout or is purposely removed, the entire circuit is disabled or thrown out of adjustment, and if the circuit is to continue'in operation without one or more of the tubes, rewiring is necessary.

It is a particular object of the invention to venable the insertion into orremoval from such a circuit of oneor more stages ofam'- plification while maintainingV the adjustments and supply voltages to the'remaining part of the set undisturbed.

In accordance with this invention, unitary switching` means are provided for cutting in or out, one or more of the stages .of the set in order to vary byrelatively large amounts the amplification produced. This switching means is arranged to substitute f fixed resistance units tor the lament `and space resistaiices of the eliminated tube,

which allows the switching to be done without substantiallyv disturbing current and voltage conditions in the remainder of the the switches in such a manner that the cal pacity between the plates of the two tubes caused by the switches is a minimum. Each switch has only a small capacity between its contacts and astwo of thesecapacities are in series between the plate circuits of the two tubes, the resulting capacity is decreased vby one-half, thus minimizing deleterious feed back effects.

There the vacuum tubes are energized from alternating current sources such as houseA lighting circuits, it is necessary to rectify the alternating current so as to obtain the direct current for the anode circuits of the tubes. It is Jfrequently desirable to eliminate all batteries, in which case the` grid biasing potential must be obtained in soine manner such as by the use of a resistance in the space current circuit. This method has the advantage of tending toward stabilizing the space current when fluctuations occur in the value of the alternating potential supplied, maintaining thereby more nearly optimum operating conditions for the vacuum tubes.

while the filaments of' the remaining tubes are heatedA bythe rectifiedl current of' which the space or plate current of the above amplifier tube may form a part. Differentxvalues of direct current voltages are obtained for the individual amplifying tubesby a:

combination. of resistances of varying values'. This direct current potentialf is filtered through an induct'ance-condenserV filter di'- rectly connected in the output of'the rectifier circuit.

The power for this circuit' is supplied througl'i an alternating current transformer 5' and a'Y rectifier tube 6of the type'commercia'lly referredl to as aY power tube. The output from the rectifier is passed'through a condenser-i'nduct'ance` filter composed' of elements l5 for smoothing out the uni-directionalV potentialy for the various tubes. From the point 8 in this filter the supply circuit divides, one branch going through the plate andI filament ofil the last amplifier tube', an'df thel other passing through resistiancei elements-l2 and 13 which are so proportioned that' the proper f value of" heating current is supplied to the filaments'of the remaining; tubes and: the proper: value of pl'ate Voltage is obtained" for these tubes. Ehe filament heating current fon the last tubel is suppliedy through. separate secondary winding 70 of the transformer 5;

As shown, potentialsfromrtheantenna 20 areimpressedf on thegrid... of) a radio frequency amplifier tube 21 through tuned cir.- cuit 22, which comprises the firststage of radio frequency amplification. From the anode of tube 2l, the signals may be impressed. onv the grid. of. tube 25 through. a

Y radio frequency transform er 26, andV a tuned circuit 27. With the circuit in the condition illustrated, however, this second'. stageA of radio frequency amplification is removed and signals from the firstradio frequency stage are applied directly t'o the detector circuit.

Associated with this second-stage of radio frequency. amplification is a unitary switch or key A-y composed.I ofl contacts numbered from 30 to 39, inclusive, the operation of which is hereinafter described. A -resi'stance 28 of approximately 50,000 ohms, or the directY current pl'ateicircuit resistance of the tube 25, is shunted across the platecircuit by contact 85, and a resistance 29 of approximately 50 ohms comparable to the resistance of the filament of tube 25 is shuntcd across the filament of this tube by the make-before-break contacts 36 and 37.

From the amplifier 25, when the latter is in circuit, the signals are fed through a radio frequency transformer 42 and a tuned circuit 48 into the detector tube 44 where they' arerectifiedin the well-known manner. The audio frequency signals may then be impressed on the grid of tube 46 through an audiofrequency transformer 47', although inthe condition of the circuit shown transformer 47 and tube 46 are outy ofy circuit. In either circuit condition the signals are next applied to audio frequen-cy transformer 66 and, tube 65. ofi the succeeding stage of audio frequency amplification, whence they pass into anoutput' circuit through the transformer 671 To switchV in; or out this first stageof audio frequency amplification there is provided aunitary switch or key B, similar to'switch'A, composed of contacts numberedA from- 50 to 59, inclusive. Resistances 6I and'V 62 of approximately 65,000 and 50 ohms, respectively, are similar to the resistances 28 and-29and are associated with the tube 46 in the same manner.

the condition ofi thev switches A- and Bindioatedf in the drawing, the'sec'ond stage ofl radio frequency and thev first stage of audio frequency amplification areelimi-nated from the circuit, as noted' above. The filament current circuit, forV all4 tubes except the las-t, may be tracedE from the filament of the-rectifier tube 6, through' the secondary winding of'transfor-mer 5, through the two filterA coils 15'- topoint- S where it divides into parallel paths, one pathA through the primary winding of transformer 67, to the `plate of amplifier tube 65, to the filament of this-tube, tothemid'point of the secondary winding 70, to point marked l1; while the other path is through the resistances l2 and 13 to point 1l from which point the combined currents pass through resistance element 14, contact 57, resistance 62,. comparable to'filament resistance of tube 46, contacts- 36 and 37, resistance 29, comparable to the filament resistance of tube 25, filament of" tube 2l", filament of tube 44, conductor 7l, to the plate and grid" and thence to the filament of rectifier tube 6 from whence it started; Since this circuit supplies the filament current for the tubes 2l, 25, 44v and 46 and the plate circuit of the last audio frequency amplifier tube, the direct current and`consequently, the direct currentV resist- `ance in this circuit should be maintained substantially constantfor both positions of switches A or Bl This is accomplished bv making the values of the replacing resistances 29 and 62' similar to the values of' the resistances of the filament of tubes and 46, respectively. It will be noted that the contacts 36 and 37 of switch yA'and the contacts 56`and4 57 of switch B aremakebefore-break arrangements adapted to pre-- vvent the breaking of this circuit'during the switching operation with the attendant troublesvwhicli would be ypresent due to the available high voltage from the rectifier which would become effective at the contacts if this circuit were broken.

The grid potential of tube is supplied from point 9 in the rectifier circuit, the value of the bias being dependent upon the fila* nient current and also upon the sum of the resistances o f the first four tubes and the resistance element 14, which may be approximately 300 ohms, to obtain the proper amount of negative potential on the last tube. Resistance 14 furtherV provides the correct amount of plate potential for the detector tube 44, the circuit, when tube 46 is eliminated, being from point 11 through conductor 73, Contact 53, primarywinding of the audio frequency transformer 66, contact 59, contact 51 to the plate.y The circuit when tube 46 is operating is from point 11 through conductor 73, Contact 52, primary winding of audio frequency transformer 47, contact 50 to the plate, comprising substantially` the saine values of resistance as when tube 46 is eliminated. It is to be noted that two contacts, are included in thepresent circuit between the tubes 46 and 44 when tube 46 is in y vContact 31, to the plate of tube `21. The se! ries capacity arrangement of the contact switch springs operates in the same manner las described for the tube 46.

operation and 'as each set of` contacts has a small capacity, the series arrangement decreases the total capacity minimizing feed back.

A resistance element of approximately 50,000 ohms and a condenser 76 Vof Vapproximately .5 microfarad are placedin the grid circuit of tube 65 to further smooth outthe ripples of the rectifier voltage for producing quiet operation and preventing interaction between the plate and grid circuits of this tube. A resistance element 77 of approximately 100,000 ohms and a condenser v7 8 of approximately .5 microfarad capacity serves the same purpose in the grid circuit of tube 46. Such a condenser resistance filter has been disclosed in my copending ap- `plicat'ion Serial No. 751,202, filed November Plate potential for the first stage audio frequency amplifier tube 46 and for'the two radio frequency amplifier tubes 21 and 25, is obtained from point 10, the junction of resistances 12 and 13 which maybe of the order of 9000 and 1500 ohms, respectively.

For the plate of tube 46, when the tube is operating, the circuit follows conductor 74 through contact 54, primary windingy of the audio frequency transformer 66, contact 58, the plate of tube Y46, filament of this tube to a point 80. .Vhen the tube is eliminated,

liovveveig-y the circuit follows conductor 74 through contact-55, resistance element 61 to thepoint 80. The throwing of the switch `B therefore, substitutes the resistance element 61which has a resistance of approxi!l supplied from point 10 as shown above.

lien this tubef is employed, the circuit passes through conductor 7 4, contact 34, pri-4 mary. winding of radio frequency transforin- .er 42, ,contact 38, plate of tube 25, the filament-of this tube, to a point 81. Vhen the tube is` not employed, the circuit traces through conductor 74 to contact 35 through resistance 28, tothe point 81. The resistances of these two circuits are approximately equal and the circuit conditions are therefore maintained substantially uniform in either position of the switch A.

Plate potential for the tube 21 is obtained from point 10 and when tube 25 is in operation, the circuit is through conductor 74, contact 32, primary winding of the radio frequency transformer 26, contact 30 to the plate of tube 21. When tube 25 has been eliminated, the circuit follows conductor 74,'througlivcontact 33, primary winding of radiofrequency transformer 42, contact 39,

In accordance with another feature of the invention a circuit arrangement is provided which `automatically maintains the tubes at approximately the proper operating condi- 'tions' asf regards plate and grid voltages under either of two conditions which are conikmonly encountered in practice. One of these' two conditions is that of variations in the supply voltage, such as variations in the line voltage, in the case of circuits deriving their energy from a `house lighting or other supply as specifically illustrated in the drawing.y

The other conditionl referred to is that of replacing a tube when it 'is burnedout or for any other reason; The manner in which this automatic regulation is secured will now be briefly explained.

In the case of a variation in the supply voltage, the plate potential ofthe last tube 65 will change, tending to produce a correspondiiigv change inV thei plate current and consequentlyin the current through resist-kr ances 12, 13 and 14 and the filaments of the first tubes. This change in .the current through the resistances produces ya change in the grid biasing potential for the tube 65 in such direction and by suchen amount as to tend to maintain the space current in tube 65 the same as it was before the varia-tions in 4the line voltage took place. Since the plate potential :and thefilam-ent current for the preceding tubes are derived from the circuit containing the :resistances 12 and 13, if the current through these resistances varies .at all, the filament current Will also vary in response to variations in lthe line voltage and the plate potential will tend to vary correspondingly. However, the -grid potentials of the amplifier tubes are derived from the filament heating circuit and arel dependent on the potential drop in this circuit so that as the filament current varies, the grid biasing potential Vof the amplifier tubes is changed in such direction and by such an amount as to tend tc compensate for the variations in the anode potential of these tubes. lvhen a tube is replaced by another tube having a different space current resistance, the substituted tube will tend to draw a lgreater or less amount of current from the spacecurrent supply circuit. However, the space current supply circuit includes resistances l2 yand 13 through one or both of which the vincreased or decreased amount of current must flow, resulting in an increase or decrease in the potential drop through these resistances and a corresponding decrease -or increase, respectively, in the plate potential applied to the tube. This reaction tends to cause stabilization of the plate current, which results in more uniform operation of the circuit when diEerent tubes are'employed than would usually result if the potential applied to the plate were maintained constant by the use of batteries kor some other constant potential source. The stabilizing effect just described is most pronounced when the values of the resistances through which the potentials are applied to the plates of the tubes arelarge with respect to the values of the plate-to-filament resistances of the tubes themselves and when these resistances are independently associated with the individual tubes which they serve, although somewhat smaller resistances will give approximate or partial regulation.

This invention is disclosed in connection with a specific receiving amplifying radio set but is not to be considered as limited to this' particular circuit but only by the scope of the following claims.

That is claimed is: l. ln a multi-stage tandem vacuum tube circuit comprising a plurality of vacuum tubes having a common source of voltage for energizing said tubes, switching means for eliminating certain of said intermediate tubes, said .switching means substituting for i said. leliminated tubes resistances comparabile to the space resistances of said tubes without affecting the :grid circuit connections to any of said tubes.

2. In a multi-stage tandem vacuum tube circuit comprising :a plurality of vacuum tubes having :a :common source of voltage for supplying space current and grid polarizing potential, :a :commen lsource cf y'filament current supplying said filaments in seri-es, switching means for eliminating certain of said intermedi-ate tubes,v said switching means removing land simultaneously .substituting for the filaments Tof said elinlinated tubes, resistances comparable to the resistances lof the filaments thereof, and removing and simultaneously substituting for vthe spaces of said eliminated tubes, resistances comparable to the space resistances thereof.

3. In a multi-stage vacuum tube circuit, a pow-er stage and-a plurality of preliminary stages feeding into said power stage, each stage comprising a discharge device having cathode, anode and grid ele-ments, a common source of space current for all of said stages, means for removing xone of said Astages and substituting in circuit therefor resistances equivalent to the filament .and anode circuit resistances, respectively, thereof, and means for maintaining the operating potentials of said stages substantially unaffected by said substitut-ion, comprising resistance elements 'connected in the path of flow cf the space current from said source to certain of said discharge devices, a connection for deriving grid biasing potential for certain of said stages from across lone of said .resistance elements, and cennecticns for deriving anode potentials for said other stages from across certain of said latter resistance elements.

4;. In combination with a plurality of vacuum tube stages interdependently supplied fromacom-mon energizing source, means for maintaining the respective energizing currents constantwhen one of said stages is rerrmoved, comprising means for inserting equivalent impedances in the connections 0ccupied by the filamentA and the anode-filament path, respectively, of the vacuum tube of the stageremoved.

5. A plurality of vacuum tube stages in which the respective operating voltages are mutually dependent on the current drawn from a common source of energy, means for deriving grid biasing potentials `from the currents d-rawnby said stages and means for removing one of said stages and substituting equivalent resistances .for the filament `and the plate-filament path respectively, tof the tube removed, whereby said grid biases are maintained constant.

6. A vacuum tube system Acomprising an inceining line, an outgoing line, a plurality of vacuum tubes in tandem relation and intermediate said lines 'adapted to amplify incoming signals, each of said tubes having an input and anoutput circuit and a connecting transformer, Aand means to remove at least one ofthe intermediate tubes of said system Without aecting the operation of the other of said stages to vary the degree of amplification in relatively large stages, said means comprising a unitary switch for deenergizing the iilament, substituting equivalent elements for the anode circuits and said filaments of the tubes removed, and changing the connection from the Output circuit of the tubes preceding the ones removed to the primaries of succeeding transformer,

Without altering the connections of said sys tem to said lines.

'In Witness whereof, I hereunto subscribe my name this 25th day of January A. D.,

DONALD F. WHITING. 

